Remote wireless unit having reduced power operating mode for a discrete multitone spread spectrum communications system

ABSTRACT

A remote unit for a personal wireless area network includes a receiver, an AC power supply, a battery-backup power supply and a controller. The battery-backup becomes operative when the AC power supply fails and supplied power to the receiver. The controller detects when the AC power supply fails and controls the receiver and the battery-backup power supply by invoking a sleep mode of operation. The sleep mode of operation is periodically interrupted by the controller controlling the receiver and the battery-backup power supply to enter a standby mode of operation in which the receiver scans for a CONNECT message from a base station indicating an incoming call. The controller coordinates the sleep mode and the standby mode of operations based on a frame count that is generated from an identification number of the remote unit. A highly bandwidth-efficient communications method is employed in the base station to enable it to coordinate communication with the remote unit when it changes from the sleep mode to the standby mode.

[0001] This patent application is a Continuation-In-Part of thecopending U.S. patent application by David Gibbons, et al. entitled“REMOTE WIRELESS UNIT HAVING REDUCED POWER OPERATING MODE”, Ser. No.______, filed Feb. 6, 1997, and assigned to AT&T Wireless Services Inc.(Docket: Gibbons 1-1, 2445-4340US1)

CROSS-REFERENCES TO RELATED APPLICATIONS

[0002] The invention disclosed herein is related to the co-pending U.S.patent application by Siavash Alamouti, Doug Stolarz, and Joel Becker,entitled “VERTICAL ADAPTIVE ANTENNA ARRAY FOR A DISCRETE MULTITONESPREAD SPECTRUM COMMUNICATIONS SYSTEM”, Ser. No. ______, filed on thesame day as the instant patent application, assigned to AT&T WirelessServices Inc., and incorporated herein by reference.

[0003] The invention disclosed herein is related to the copending U.S.patent application by Alamouti, et al., entitled “Method for FrequencyDivision Duplex Communications”, Ser. No. ______, filed Feb. 6, 1997,assigned to AT&T Wireless Services, and incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0004] 1. Field of the Invention

[0005] The present invention relates to improvements to communicationssystems. More particularly, the present invention relates to wirelessdiscrete multitone spread spectrum communications systems.

[0006] 2. Description of the Related Art

[0007] Wireless communications systems, such as cellular and personalcommunications systems, operate over limited spectral bandwidths andmust make highly efficient use of the scarce bandwidth resource forproviding good service to a large population of users. A Code DivisionMultiple Access (CDMA) protocol has been used by wireless communicationssystems for efficiently making use of limited bandwidths and uses aunique code for distinguishing each user's data signal from data signalsof other users. Knowledge of the unique code with which any specificinformation is transmitted permits separation and reconstruction of eachuser's message at the receiving end of the communication channel.

[0008] Adaptive beamforming technology has become a promising technologyfor wireless service providers for offering large coverage, highcapacity, and high quality service. Based on this technology, a wirelesscommunication system can improve its coverage capability, systemcapacity, and performance significantly. A personal wireless accessnetwork (PWAN) system, described in the cross-referenced Alamouti,Stolarz, et al. patent applications, uses adaptive beamforming combinedwith a form of the CDMA protocol known as discrete multitone spreadspectrum (DMT-SS) for providing efficient communications between a basestation and a plurality of remote units (RUs).

[0009] The remote units are powered primarily from AC power sources andinclude a battery for providing battery backup power when AC powerfails. To conserve battery power, an RU has a sleep mode of operationwith periodic power-up modes for checking whether any calls areattempting to be connected to the RU. When an RU is in a sleep mode, itexpedient that the system operate in such a way so that appropriateactions are taken for completing a call to a sleep mode RU.

[0010] One approach for ensuring that calls are completed to a remoteunit operating in a sleep mode is to maintain a database at a centrallocation that stores the current operating mode of each remote in thesystem. When a remote unit enters a sleep mode of operation, the remoteunit reports the change of operational status to the database.Similarly, the remote unit reports a change of status back to a standbyoperating mode. This approach has a drawback when a number of remoteunits recorded in the database experience frequent power outages. Insuch a situation, recording, managing and synchronizing power outageinformation in the database is particularly cumbersome when the databaseis large, perhaps holding status information for 3 to 4 thousand remoteunits. This drawback is further compounded when the database isduplicated multiple times throughout the system. When several thousandsubscribers experience a power outage and AC power is restored beforethe database has completed recording the power outage, a databaseapproach becomes unwieldy. Another complicated situation is whenmultiple remote units lose power at the same time. The affected remoteunits cannot all access the channel simultaneously for communicatingtheir status to the database. A collision avoidance scheme must beimplemented that spans a period of time and that is open for thepossibility of power being restored before the database has beencompletely revised.

[0011] This approach has another drawback in that a remote unit enteringthe sleep mode consumes system bandwidth in notifying the database. FIG.4 shows an exemplary flow of internal messaging that occurs betweenvarious layers of a remote unit when loss of AC power is detected and adatabase is notified of the operational status change. Time is shownalong the vertical axes of FIG. 4, with advancing time being indicatedtoward the bottom of FIG. 4. In FIG. 4, four layers of the remote unitoperating system are shown: Health; OAM&P (Operations, Administration,Maintenance & Provisioning), MAC (Media Access Control) and physical.Only MAC layer of the base station is shown. At 40, AC power failure isdetected by the Health layer. At 41, an EVENT message is sent from theHealth layer to the OAM&P layer indicating that AC power has failed. TheOAM&P layer sends an ACTION message to the MAC layer at 42. The MAClayer responds at 43 by sending an ACTION_RSP message to the OAM&P layerindicating that base station notification is pending. At 44, the MAClayer waits a random length period of time before sending an unsolicitedCAC message at 45 to the MAC layer of the base station indicating theneed for the remote unit to enter the sleep mode. At 46, the MAC layerof the base station sends an acknowledgment message to the MAC layer ofthe remote unit acknowledging receipt of the unsolicited CAC message. Inresponse, the MAC layer of the remote unit sends an EVENT message at 47to the OAM&P layer that the notification is done. The OAM&P layer firstsends an EVENT message to the MAC layer indicating that the sleep modehas been entered at 48, and then sends a message at 49 to the physicallayer to power down.

[0012] What is needed is a way for a PWAN system to be aware that aremote unit is operating in a sleep mode so that appropriate actions canbe taken by the system so that calls can be completed to a remote unitoperating in a sleep mode.

SUMMARY OF THE INVENTION

[0013] The present invention provides a method for reducing powerconsumption of a remote unit in a PWAN system. A remote unit is poweredusing a battery backup power supply when an AC power supply fails at theremote unit. A sleep mode of operation is entered at the remote unitthat has a reduced power consumption for the battery backup powersupply. The remote unit is synchronized to a TDD timing structure apredetermined period of time after entering the sleep mode of operation.A standby mode of operation is then entered at the remote unit in whicha CONNECT message indicating an incoming call for the remote unit isscanned for by the receiver. When no CONNECT message is received, theremote unit reenters the sleep mode of operation. According to theinvention, the predetermined period of time is a predetermined number ofsubframes after a boundary subframe of the TDD timing structure.Preferably, the predetermined number of subframes is based on anidentification number of the remote unit.

[0014] The present invention also provides a remote unit for a personalwireless area network that includes a receiver, an AC power supply, abattery-backup power supply and a controller. The battery-backup becomesoperative when the AC power supply fails and supplies power to thereceiver. The controller detects when the AC power supply fails andcontrols the receiver and the battery-backup power supply by invoking asleep mode of operation. The sleep mode of operation is periodicallyinterrupted by the controller controlling the receiver and thebattery-backup power supply to enter a standby mode of operation inwhich the receiver scans a CONNECT message indicating an incoming call.The controller coordinates the sleep mode and the standby mode ofoperations based on a frame count that is generated from anidentification number of the remote unit.

[0015] In accordance with another aspect of the invention, a highlybandwidth-efficient communications method is disclosed for the basestation to enable it to communicate with a remote unit that is in thesleep mode. The remote unit has a unique identification value that isdifferent from the identification value of other remote units that maybe communicating with the base station. The base station begins byestablishing a periodic reference instant at the base station and at theremote station. Then the base station determines a delay intervalfollowing the periodic reference instant at the base station, the delayinterval being derived from the unique identification value of theremote unit. The base station receives spread signals from the remoteunits with which it communicates, each comprising an incoming datatraffic signal spread over a plurality of discrete traffic frequencies.The base station adaptively despreads the signals received it receivesby using despreading weights. The base station attempts to initiate acommunication with the remote unit that is currently in the sleep mode.If the attempting step fails to initiate communications with the remoteunit, the base station concludes that the remote unit is in the sleepmode. In response to this, the base station waits for the delay intervalfollowing the periodic reference instant at the base station beforetransmitting to the remote unit. The base station then transmits to theremote unit a spread signal comprising an outgoing data traffic signalspread over a plurality of discrete traffic frequencies. The remote unithas simultaneously changed from the sleep mode to the standby mode andis able to receive and respond to the spread signal transmitted from thebase station.

[0016] In accordance additional aspects of the invention, the basestation is part of a wireless discrete multitone spread spectrumcommunications system. Further, the periodic reference instant isestablished by a beginning subframe count instant that is incremented bya packet count value at the base station and at the remote unit. Inaddition, the delay interval is determined by a value N of a quantity ofM least significant bits of the unique identification value of theremote unit, the delay interval being an interval required for theoccurrence of a plurality of N of the beginning subframe count instants.The resulting invention enables the base station to be aware that aremote unit is operating in a sleep mode so that appropriate actions canbe taken by the base station to assure that calls can be completed tothe remote unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention is illustrated by way of example and notlimitation in the accompanying figures in which like reference numeralsindicate similar elements and in which:

[0018]FIG. 1 is an architectural diagram of the PWAN system, includingremote stations transmitting to a base station;

[0019]FIG. 2 is an architectural diagram of the remote station X as asender;

[0020]FIG. 3 is an architectural diagram of the remote station X as areceiver;

[0021]FIG. 4 shows an exemplary messaging flow occurring between variouslayers of an exemplary remote unit and through an airlink to a basestation when a loss of AC power at the remote unit is detected;

[0022]FIG. 5 shows a message flow sequence for a terminating call forthe situation when a target remote unit is operating in the sleep mode;

[0023]FIG. 6 shows a sequence of events with respect 6 ms subframestructure of the present invention;

[0024]FIG. 7 is an exemplary graph showing Battery Operating Time,measured in hours, for Sleep Mode Duty Cycle (:1); and

[0025]FIG. 8 is an architectural diagram of the base station Z.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026]FIG. 1 shows an architectural diagram of the personal wirelessaccess network (PWAN) system described in the referenced Alamouti,Stolarz, et al. patent applications and which is the environment of thepresent invention. Two users, Alice and Bob, are located at a remotestation unit, or remote unit (RU), X and wish to transmit theirrespective data messages to a base station Z. Remote unit X ispositioned to be equidistant from each of antenna elements A, B, C, andD at base station Z. Two other users, Chuck and Dave, are located at aremote station unit Y and also wish to transmit their respective datamessages to base station Z. Remote unit Y is geographically differentfrom remote unit X and is not equidistant from each of antenna elementsA, B, C, and D of base station Z. Remote units X and Y, and base stationZ use a form of the CDMA protocol known as discrete multitone spreadspectrum (DMT-SS) which is used for providing efficient communicationsbetween base stations and remote units. The DMT-SS protocol is indicatedin FIG. 1 as a multi-tone CDMA.

[0027] In the DMT-SS protocol, a user data signal is modulated by a setof weighted discrete frequencies or tones. The weights are spreadingweights that distribute the data signal over many discrete tonescovering a broad range of frequencies. The weights are complex numbershaving a real component that is used for modulating the amplitude of atone and a complex component that is used for modulating the phase ofthe same tone. Each tone in the weighted-tone set bears the same datasignal. Plural users at a transmitting station can use the same tone setfor transmitting their data, but each of the users sharing the tone sethas a different set of spreading weights. The weighted-tone set for aparticular user is transmitted to the receiving station where it isprocessed with despreading weights that are related to the user'sspreading weights for recovering the user's data signal. For each of aplurality of spatially separated antennas at the receiver, the receivedmultitone signals are transformed from time-domain signals tofrequency-domain signals. Despreading weights are assigned to eachfrequency component of the signals that are received by each antennaelement. The values of the despreading weights are combined with thereceived signals for obtaining an optimized approximation of individualtransmitted signals characterized by a particular multitone set andtransmitting location.

[0028] The PWAN system has a total of 2560 discrete tones (carriers)that are equally spaced in 8 MHZ of available bandwidth in the frequencyrange of 1850 to 1990 MHZ, with a spacing between the tones of 3.125KHz. The tones are used for carrying traffic messages and overheadmessages between the base station and the plurality of remote units. Thetotal set of tones are numbered consecutively from 0 to 2559, startingfrom the lowest frequency tone. The tones used for traffic messages aredivided into 32 traffic partitions, with each traffic channel requiringat least one traffic partition of 72 tones.

[0029] The overhead message tones are used for establishingsynchronization and for passing control information between basestations and remote units. A Common Link Channel (CLC) is used by a basestation for transmitting control information to remote units. A CommonAccess Channel (CAC) is used by a remote unit for transmitting messagesto the base station. There is one grouping of tones assigned to eachchannel. The overhead channels are used in common by all remote unitswhen control messages are exchanged with a base station.

[0030] Transmission from a base station to a remote unit is called“forward transmission” and transmission from a remote unit to a basestation is called “reverse transmission”. Time Division Duplexing (TDD)is used by base stations and remote units for transmitting data andcontrol information in both directions over the same multi-tonefrequency channel. The time between recurrent transmissions in eitherdirection is called a TDD period which, is equal to 3 ms. For every TDDperiod, there are four consecutive transmission bursts in eachdirection. Data is transmitted during each burst using multiple tones.The base station and each remote unit synchronize and conform to a TDDtiming structure and framing structure that has 1 frame equal to 8subframes and 1 subframe equal to 2 TDD periods. A superframe is 256subframes, or 1536 ms. All remote units and base stations aresynchronized such that all remote units transmit simultaneously and thenall base stations transmit simultaneously. When a remote unit initiallypowers up, it acquires synchronization from a base station so thatcontrol and traffic messages can be exchanged within the prescribed TDDtime format. A remote unit must also acquire frequency and phasesynchronization for the DMT-SS signals so that the remote unit isoperating at the same frequency and phase as an associated base station.

[0031] Selected tones within each tone set are designated as pilot tonesthat are distributed throughout the frequency band and carry known datapatterns for enabling an accurate channel estimation. A series of pilottones, having known amplitudes and phases, are spaced apart in frequencyby approximately 30 KHz for providing an accurate representation of achannel response over the entire transmission band, that is, theamplitude and phase distortion introduced by the communication channelcharacteristics over the transmission band.

[0032]FIG. 2 shows an architectural diagram of remote station Xoperating as a sender station. Alice and Bob each input data to remotestation X. The data is sent to a vector formation buffer 202 and also toa cyclic redundancy code generator 204. Data vectors are output frombuffer 202 to a trellis encoder 206. The data vectors are in the form ofa data message formed by concatenating a 64 K-bit data block with aserially assigned block number. CRC generator 204 generates LCC vectorsthat are output to trellis encoder 206. The LCC vectors are in the formof an error detection message formed by concatenating a CRC value withthe serially assigned block number of the data block. The trellisencoded data vectors and LCC vectors are then output to a spectralspreading processor 208. The resultant data tones and LCC tones are thenoutput from processor 208 to a transmitter 210 for transmission to thebase station.

[0033] The personal wireless access network (PWAN) system described inthe cross-referenced Alamouti, Stolarz, et al. patent applicationprovides a more detailed description of a high-capacity mode, where onetraffic partition is used in one traffic channel. A base stationtransmits information to multiple remote units that are located in thebase station's cell. The transmission formats are for a 64 Kbps trafficchannel, together with a 4 Kbps Link Control Channel (LCC) between thebase station and a remote unit. A binary source, for example, Alice orBob, delivers data, or information bits, to a sender transmitter at 64Kbits/sec. This translates to 48 bits in one transmission burst. Theinformation bits are encrypted according to a triple data encryptionstandard (DES) algorithm. The encrypted bits are then randomized in adata randomization block. A bit-to-octal conversion block converts therandomized binary sequence into a sequence of 3-bit symbols. The symbolsequence is converted into 16 symbol vectors. The term vector generallyrefers to a column vector, which is generally complex. One symbol fromthe LCC is added to form a vector of 17 symbols.

[0034] The 17-symbol vector is trellis encoded starting with the mostsignificant symbol (first element of the vector) and is continuedsequentially until the last element of the vector (the LCC symbol). Thisprocess employs convolutional encoding for converting the input symbol(an integer between 0 and 7) to another symbol (between 0 and 15) andmaps the encoded symbol to its corresponding 16 QAM (or 16 PSK) signalconstellation point. The output of the trellis encoder is therefore avector of 17 elements where each element is a signal within a set of 16QAM (or 16 PSK) constellation signals. (The term signal will generallyrefer to a signal constellation point.)

[0035] A link maintenance pilot signal (LMP) is added to form an18-signal vector, with the LMP as the first element of the vector. Theresulting (18×1) vector is pre-multiplied by a (18×18) forward smearingmatrix yielding an (18×1) vector b. Vector b is element-wise multipliedby an (18×1) gain preemphasis vector yielding another (18×1) vector c.Vector c is post-multiplied by a (1×32) forward spatial and spectralspreading vector yielding a (18×32) matrix R(p), where p denotes thetraffic channel index and is an integer. The 32 columns of matrix Rresults from multiplying the spectral spreading factor 4 and spatialspreading factor 8. The (18×32) matrices corresponding to all trafficchannels carried (on the same traffic partition) are then combined(added) for producing a resulting 18×32 matrix S.

[0036] Matrix S is partitioned by groups of four columns into eight(18×4) submatrices A₀ to A₇. The indices 0 to 7 of submatrices A₀ to A₇correspond to the antenna elements over which these symbols willeventually be transmitted. Each submatrix is mapped to tones within onetraffic partition. A lower physical layer places the baseband signals indiscrete Fourier transfer (DFT) frequency bins where the data isconverted into the time-domain and sent to its corresponding antennaelements (0 to 7) for transmission. This process is repeated from thestart for the next 48 bits of binary data to be transmitted in the nextforward transmission burst.

[0037]FIG. 3 is an architectural block diagram of remote station Xoperating as a receiving station. Data tones and LCC tones are receivedby remote station antenna X and a receiver 610. Receiver 610 passes thedata tones and the LCC tones to a spectral despreading processor 612which despreads the data tones and LCC tones. The despread signals arethen output from processor 612 to a trellis decoder 614. Trellis decoder614 generates data vectors from the despread signals. The data vectorsare then output to a vector disassembly buffer 616. Data for Alice anddata to Bob are output from buffer 616 to Alice and Bob, respectively.Data for Alice and Bob are also input to a CRC generator 618. CRCgenerator 618 computes a new CRC value for every 64 K-bit data block andoutputs the new CRC value with the block number to a buffer within a CRCcomparison processor 620. The receiving station buffers error detectionmessages that are received from the link control channel in CRCcomparison processor 620 so that the error detection messages areaccessible by their block numbers N, N+1, N+2, etc. When the receivingstation receives a data message on the traffic channel, it performs aCRC calculation on the data block in the message with CRC generator 618for obtaining a resulting new CRC value. If the comparison determinesthat there is a difference in the values, then an error signal isgenerated by an error signal generator 622. The error signal can beprocessed and used in several ways by an error processor 630. Forexample, the error signal can initiate a negative acknowledgment signalthat is to be sent from the receiving station back to the sender stationrequesting that the sender repeat transmission of the data block. Theerror signal can also initiate an update in spreading and despreadingweights at the receiving station for improving thesignal-to-interference and noise ratio of the traffic channel. Anotheruse of the error signal is for initiating an alarm used for other realtime control. Yet another use of the error signal is as part of alogging signal for compilation of a long term report relating to trafficchannel quality.

[0038] According to the invention, a remote unit includes a standby modeof operation and a sleep mode of operation. Normally, the standby modeis the mode in which a remote unit scans the CLC channel for a CONNECTmessage for the remote unit. The sleep mode of operation provides areduced power consumption operating mode for extending remote unitbattery runtime during an AC power outage condition. During the sleepmode of operation, the remote unit periodically switches between thestandby mode and sleep mode, with the overall effect being a reductionin the average power required by the remote unit.

[0039] Delivery of a CONNECT message to a remote unit operating in thesleep mode is scheduled so that the remote unit is in the standbyportion of the sleep mode. That is, the remote unit is synchronized andready for receiving data from the CLC when the base station beginstransmitting on the CLC. In order to achieve synchronization, a systemwide Packet Count (PKT_CNT) is used. The basic unit of measure forsynchronization is a mod[8] PKT_CNT, which is called a subframe count(SUBFRM_CNT). The SUBFRM_CNT is incremented every 256 PKT_CNTs, or every6 ms.

[0040] The base station and the remote unit both preferably use theleast significant 8 bits of the remote unit ID for determining theparticular SUBFRM_CNT at which the CLC CONNECT message should be sent tothe remote unit and, simultaneously, the appropriate time at which theremote unit should be in the standby portion of the sleep mode forreceiving the CONNECT message. When the least significant 8 bits of theremote unit ID are used, the remote unit enters the standby mode onceevery 256 subframes and is ready for receiving an incoming call. Theparticular subframe that a remote unit will be ready for receiving anincoming call is called the N_(listen) for the remote unit.

[0041] To avoid using a remote unit power status database that ismaintained at a central location, the sleep mode features of the presentinvention are preferably implemented as part of a standard terminatingcall retry mechanism. That is, when a terminating call request isreceived at the base station MAC Layer, the MAC Layer Access Managerproceeds normally through a terminating call setup procedure bytransmitting a CONNECT message on the CLC to the target remote unit. Inthe situation when the target remote unit is operating in the sleep modeat the time of the CONNECT message transmission, the remote unit willgenerally be unable to process the message. The base station MAC LayerAccess Manager will time-out and retry transmission of the CONNECTmessage. Preferably, a retry timer T_(r) is nominally set to 72 ms. Thebase station MAC Layer Access Manager retries the CONNECT message for apredetermined number of tries that is set by a system manager.Preferably, the retry count is 2.

[0042] When the number of retries equals the retry count, the basestation MAC Layer Access Manager determines that the remote unit is inthe sleep mode and, consequently, attempts to deliver the CONNECTmessage at a scheduled time that is based on the target remote unit ID.The scheduled time is a subframe occurring N_(listen) subframes afterthe boundary subframe for the TDD timing structure.

[0043] The base station MAC Layer Access Manager also reserves the CLCslot(s) required for completing the CLC CONNECT message transmission atthe time the N_(listen) subframe number is derived. That is, when thebase station MAC Layer Access Manager has reached its retry count for aCONNECT message and has determined the N_(listen) subframe, CLC slotavailability is examined for reserving the appropriate CLC slot(s) foruse. As an alternative, a remote unit can scan up to 3 CLC slots for aCONNECT message when in the sleep mode so that a base station can selectfrom 3 CLC slots in case a specific slot is unavailable.

[0044]FIG. 5 shows a message flow sequence for a terminating call forthe situation when a target remote unit is operating in the sleep mode.The MAC Layer of the base station receives a terminating call request at50. At 51, the MAC Layer of the base station sends a CLC CONNECT messageto the target remote unit. Since the remote unit is in the sleep mode,it does not receive the CLC CONNECT message and, therefore, does notrespond. Since there is no response from the target remote unit duringthe T_(retry) period 52, the MAC Layer of the base station sends asecond CLC CONNECT message to the target remote unit at 53. The remoteunit does not respond during T_(retry) 54, so the MAC Layer of the basestation determines the N_(listen) subframe for the remote unit using theleast significant 8 bits of the remote unit ID and waits for theparticular N_(listen) subframe at 55. At N_(listen) for the remote unit,the MAC Layer of the base station sends a CLC CONNECT message at 56. AtN_(listen), the remote unit is in the standby mode and ready to receivethe CLC CONNECT message at 57. In response, the remote unit MAC Layersends a CAC_ACK message to the base station at 58.

[0045] The following definitions are used for describing the sleep modeof operation of the present invention: T_(sleep) = the time that aremote unit is in a low-power mode (i.e., sleeping). T_(sync) = the timerequired by a remote unit for re-acquiring synchronization when exitingthe sleep mode. T_(scan) _(—) _(clc) = the time that a remote unit isoperating in a standby mode scanning the CLC for a CONNECT message.T_(standby) = the total time a remote unit is running (i.e., T_(sync) +T_(scan) _(—) _(clc)) D_(sleep) = T_(sleep) + T_(standby))/T_(standby),that is, the definition of the duty cycle of the sleep mode duty cycle.

[0046] Since the base station transmits the CONNECT message at theN_(listen) subframe so that the call can be completed, and the remoteunit therefore must be ready for receiving the messages on the CLCchannel at the N_(listen) subframe. The remote unit MAC Layer AccessManager is capable of deriving the N_(start) _(—) _(sync) subframenumber and insures that all hardware required for the remote unitsynchronization and CLC scanning efforts are released from sleep mode atthat time. This is done, for example, by using a programmable hardwarecounter 640 that is clocked in synchronism with the TDD subframe of thesystem, as shown in FIG. 3. Prior to entering the sleep mode, or at thetime the sleep mode is entered, CPU 650 preferably uses the leastsignificant 8 bits of the remote unit ID for determining the N_(listen)subframe for the remote unit. CPU 650 loads counter 640 with a valuerelated to N_(listen) and synchronizes counter 640 using a Start Syncsignal. Counter 640 provides an interrupt to CPU 650 once every 256subframes, initiating a re-synchronization process. CPU 650 responds bycontrolling power supply 660 to provide power 661 to the variouscomponents of remote unit used for receiving a CLC CONNECT message. CPU650 also outputs an enabling signal to the spectral despreadingprocessor 612 to enable the remote unit to receive messages from thebase station.

[0047] The remote unit begins its re-synchronization effort at asubframe N_(start) _(—) _(sync) that occurs some determined period oftime prior to the occurrence of the N_(listen) subframe. Simulations ofthe remote unit synchronization algorithms indicate that a remote unitacquires synchronization with a base station when exiting a period ofsleep in a minimum time of 122 ms and a maximum time of 200 ms. Theactual time additionally depends on hardware component tolerances, theambient temperature and numerous other factors. For the purposes of thisdisclosure, a worst case synchronization acquisition time T_(sync) of200 ms is used. This equates to approximately 34 subframes. Therefore,N_(start) _(—) _(=N) _(listen)−34 subframes.

[0048]FIG. 6 is a timing diagram showing the sequence of events for aremote unit operating in the sleep mode. Each vertical line in FIG. 6represents a subframe boundary. The time between each subframe boundaryis 6 ms. A remote unit is shown as being in a sleep mode. At N_(start)_(—) _(sync), counter 640 sends an interrupt request to CPU 650 (FIG.3). CPU 650 responds by controlling power supply 660 to provide power tothe various components of the remote unit needed for receiving a CLCCONNECT message. In FIG. 6, the remote unit is in the sleep mode at 60.At 61, N_(start) _(—) _(sync) occurs and the remote unit resynchronizesfor a number of subframes. Preferably, about 34 subframes are requiredfor a remote unit to reacquire synchronization. At N_(listen), theremote unit scans the CLC channel for any CLC CONNECT messages for theremote unit. The remote unit scans for 2 subframes, as shown in FIG. 6at 62. The remote unit can also be set to scan for a CLC CONNECT messageover a different number of subframes other than 2 subframes dependingupon system requirements. If no CLC CONNECT message is received atN_(listen), the remote unit returns to the sleep mode at 63. If a CLCCONNECT message is received, the call is established in a normal manner.

[0049] As a first illustrative example of the timing aspects of thesleep mode of the present invention, the least significant 8 bits of aremote unit ID are used so that the N_(listen) cycle time is 1536 ms(256×6 ms). The remote unit synchronization acquisition time N_(sync) isestimated to be 34 subframes (204 ms), and a CLC scan time for 2 CLCsubframes is chosen. It follows that,

[0050] T_(sleep)=220 subframe times=220×6 ms=1.320 s

[0051] T_(sync)=204 ms=34 subframes×6 ms

[0052] T_(scan) _(—) _(clc)=12 ms=2 subframes×6 ms

[0053] T_(standby)=212 ms

[0054] Therefore, the total sleep mode/standby mode cycle time is 1536ms, and the total remote unit power-on time is 212 ms. The overall dutycycle is 7.25:1. For this example, the maximum delay for delivery of aCONNECT message is 1.530 seconds (1536 ms−6 ms). The nominal CONNECTmessage delay delivery time is about 0.766 seconds.

[0055] Using a longer delay in CONNECT message delivery time permits theremote unit to be in the sleep mode for a greater period of time. Asanother example, the N_(listen) subframe is determined by using theleast significant 9-bits of a remote unit ID. Thus, the N_(listen)interval is 512 subframes. In this example, even though the sleep timeis longer, the maximum synchronization acquisition time T_(sync) remainsthe same. This is based on the fact that any temperature change of theremote unit is not sufficient for requiring a coarse TDD synchronizationto be performed. It follows that,

[0056] T_(sleep)=476 subframe times=476×6 ms=2.856 s

[0057] T_(sync)=204 ms=34 subframes×6 ms

[0058] T_(scan) _(—) _(clc)=12 ms=2 subframes×6 ms

[0059] T_(standby)=212 ms

[0060] The total sleep mode/standby mode cycle time is 3072 ms (512×6ms), and the total remote unit power-on time is 212 ms. The overall dutycycle is 14.5:1. For this example, the maximum delay of delivery of aCONNECT message is 3.066 seconds (3072 ms−6 ms). The nominal time fordelivery of a CONNECT message is about 1.536 s.

[0061] Table I below summarizes various scenarios: TABLE I Nominal CLCBattery Sleep CONNECT RU RU Power Runtime Time message delaySynchronization Duty Cycle (approx. (ms) time (ms) Time (ms) (approx.)hrs) 1320  663 220 7.25:1   11.5 1320  663 150  9:1 12 2856 1428 15018:1 13.5 2856 1428 200 14:5 13 2856 1428 300 10:1 12

[0062] The situation of a call originating from a remote unit that isoperating in the sleep mode is straight forward compared to thesituation when a call terminates at a sleeping remote unit. That is, theremote unit exits the sleep mode in response to a user command. Theoriginating call delivery time, i.e., the time taken for delivering anACCESS message on the CAC, is delayed by approximately 200 ms since theremote unit must re-acquire synchronization before the ACCESS messagemay be transmitted.

[0063] In normal system operation, a base station polls remote units ata periodic rate for determining status of each remote unit. Each remoteunit responds to the Poll Request message with a Poll response messageusing the CAC channel. When a remote unit is in a sleep mode ofoperation, the Poll Request message will not be received and,consequently, the remote unit will not respond with a Poll Responsemessage. The present invention provides two alternatives for handlingsuch a situation from the system point of view. The first approach is toalways schedule a Poll Request message to arrive at a remote unit duringthe N_(listen) subframe for the remote unit whether the remote is in thestandby or the sleep mode. The remote unit will receive the Poll Requestmessage regardless of AC power status. A disadvantage associated withthis approach is that the CAC channel is used by the remote unit for aPoll Response message, causing the remote unit transmitter to be used,effectively wasting battery power when in the sleep mode.

[0064] The alternative approach is for a remote unit to ignore the Pollmessage from the base station during AC power outage situations andallow an OAM&P Layer at the base station to recognize that anon-responsive remote unit may possibly be in the sleep mode and,consequently, be aware of the power status of the remote unit inquestions power.

[0065]FIG. 7 is an exemplary graph showing Battery Operating Time,measured in hours, for Sleep Mode Duty Cycle (:1). From FIG. 7, it isapparent that the length of time that a remote unit is sleeping has asignificant impact on the run time of the battery. Also, from FIG. 7, itis also apparent that the battery run time begins to flatten with dutycycle after about a 10:1 ratio. Lab results for simulated sleep modeoperation with a new, 7.2 amp-hour battery installed in a prototypeuninterruptable power supply have yielded runtimes between 12 hours, 12minutes to 12 hours, 32 minutes under the conditions that the remoteunit is at room temperature, the sleep mode period is set for 3 seconds,and the sleep mode duty cycle is 10:1 (0.3 s standby state and a 2.7 ssleep state).

[0066] A remote unit operating in the sleep mode preferably provides thefollowing characteristics:

[0067] Sleep time=2856 ms

[0068] RU Synchronization Time=200 ms

[0069] Call delivery delay=1428 ms nominally

[0070] RU CLC Scan time=36 ms (i.e., three slots for flexibility at BaseMAC Layer)

[0071] Total Cycle Time=3092 ms

[0072] Standby Time=236 ms

[0073] Duty Cycle=13:1 (approx.)

[0074] Battery Operating Time=12.5 hours (approx.)

[0075]FIG. 8 is an architectural diagram of the base station as asender. The PSTN inputs data to base station Z. The data is sent to thevector formation buffer 502 and also to the cyclic redundancy codegenerator 504. Data vectors are output from buffer 502 to the trellisencoder 506. The data vectors are in the form of a data message formedby concatenating a 64 K-bit data block with its serially assigned blocknumber. The LCC vectors output from the CRC generator 504 to the trellisencoder 506 are in the form of an error detection message formed byconcatenating the CRC value with the block number. The trellis encodeddata vectors and LCC vectors are then output to the spectral and spatialspreading processor 508. The resultant data tones and LCC tones are thenoutput from processor 508 to the transmitter 210 for transmission to theremote station.

[0076] The base station transmits the CONNECT message at the N_(listen)subframe so that the call can be completed to the remote unit. The basestation knows to send the messages on the CLC channel at the N_(listen)subframe. The base station's MAC Layer Access Manager is capable ofderiving the N_(start) _(—) _(sync) subframe number. This is done, forexample, by using a programmable hardware counter 540 that is clocked insynchronism with the TDD subframe of the system, as shown in FIG. 8.When the base station wants to send a message to the remote unit, theCPU 550 preferably uses the least significant 8 bits of the remote unitID for determining the N_(listen) subframe for the remote unit. CPU 550loads counter 540 with a value related to N_(listen) and synchronizescounter 540 using a Start Sync signal. Counter 540 provides an interruptto CPU 550 once every 256 subframes, initiating a re-synchronizationprocess. CPU 550 responds by outputting an enabling signal to thespectral and spatial spreading processor 508 to enable the base stationto transmit messages to the remote unit when the remote unit is in itsstandby mode.

[0077] Still another alternate embodiment applies the above describedinvention in the PWAN Frequency Division Duplex Communications Systemdescribed in the Alamouti, Michaelson et al. patent application citedabove.

[0078] Although the preferred embodiments of the invention have beendescribed in detail above, it will be apparent to those of ordinaryskill in the art that obvious modifications may be made to the inventionwithout departing from its spirit or essence. Consequently, thepreceding description should be taken as illustrative and notrestrictive, and the scope of the invention should be determined in viewof the following claims.

What is claimed is:
 1. In a wireless communications network, a method ina base station to communicate with a remote unit that is in a sleepmode, the remote unit having a unique identification value, comprisingthe steps of: establishing a periodic reference instant at the basestation and at the remote station; determining a delay intervalfollowing said periodic reference instant at the base station, saiddelay interval being derived from said unique identification value ofsaid remote unit; and transmitting a message from the base station tothe remote unit at a second instant following said delay interval, saidremote unit having changed from said sleep mode to a standby mode aftersaid delay interval.
 2. The method of claim 1 , wherein said basestation is part of a wireless discrete multitone spread spectrumcommunications system.
 3. The method of claim 1 , wherein said periodicreference instant is established by a beginning subframe count instantthat is incremented by a packet count value at the base station and atthe remote unit.
 4. The method of claim 3 , wherein said delay intervalis determined by a value N of a quantity of M least significant bits ofsaid unique identification value of said remote unit, the delay intervalbeing an interval required for the occurrence of a plurality of N ofsaid beginning subframe count instants.
 5. The method of claim 4 ,wherein said remote unit changes from said sleep mode to a standby modeafter said delay interval.
 6. In a wireless communications network, amethod in a base station to communicate with a remote unit that is in asleep mode, the remote unit having a unique identification value,comprising the steps of: establishing a periodic reference instant atthe base station and at the remote station; determining a delay intervalfollowing said periodic reference instant at the base station, saiddelay interval being derived from said unique identification value ofsaid remote unit; attempting to initiate a communication from said basestation to said remote unit; concluding at the base station that theremote unit is in a sleep mode if said attempting step fails to initiatecommunications with the remote unit; waiting for said delay intervalfollowing said periodic reference instant at the base station; andtransmitting a message from the base station to the remote unit at asecond instant following said delay interval, said remote unit havingchanged from said sleep mode to a standby mode after said delayinterval.
 7. The method of claim 6 , wherein said base station is partof a wireless discrete multitone spread spectrum communications system.8. The method of claim 6 , wherein said periodic reference instant isestablished by a beginning subframe count instant that is incremented bya packet count value at the base station and at the remote unit.
 9. Themethod of claim 8 , wherein said delay interval is determined by a valueN of a quantity of M least significant bits of said uniqueidentification value of said remote unit, the delay interval being aninterval required for the occurrence of a plurality of N of saidbeginning subframe count instants.
 10. The method of claim 9 , whereinsaid remote unit changes from said sleep mode to a standby mode aftersaid delay interval.
 11. A highly bandwidth-efficient communicationsmethod in a base station to communicate with a remote unit that is in asleep mode, the remote unit having a unique identification value,comprising the steps of: establishing a periodic reference instant atthe base station and at the remote station; determining a delay intervalfollowing said periodic reference instant at the base station, saiddelay interval being derived from said unique identification value ofsaid remote unit; receiving at a base station a spread signal comprisingan incoming data traffic signal spread over a plurality of discretetraffic frequencies; adaptively despreading the signals received at thebase station by using despreading weights; attempting to initiate acommunication from said base station to said remote unit; concluding atthe base station that the remote unit is in a sleep mode if saidattempting step fails to initiate communications with the remote unit;waiting for said delay interval following said periodic referenceinstant at the base station; and transmitting at the base station to theremote unit a spread signal comprising an outgoing data traffic signalspread over a plurality of discrete traffic frequencies.
 12. The methodof claim 11 , wherein said base station is part of a wireless discretemultitone spread spectrum communications system.
 13. The method of claim11 , wherein said periodic reference instant is established by abeginning subframe count instant that is incremented by a packet countvalue at the base station and at the remote unit.
 14. The method ofclaim 13 , wherein said delay interval is determined by a value N of aquantity of M least significant bits of said unique identification valueof said remote unit, the delay interval being an interval required forthe occurrence of a plurality of N of said beginning subframe countinstants.
 15. The method of claim 14 , wherein said remote unit changesfrom said sleep mode to a standby mode after said delay interval.
 16. Aremote unit for a personal wireless area network comprising: a receiver;an AC power supply coupled to the receiver and supplying power to thereceiver; a battery-backup power supply coupled to the receiver, thebattery-backup power supply becoming operative to supply power to thereceiver when the AC power supply fails; and a controller coupled to thereceiver, the AC power supply and the battery-backup power supply, thecontroller detecting when the AC power supply fails and in responsecontrols the receiver and the battery-backup power supply by invoking asleep mode of operation, the sleep mode operation being periodicallyinterrupted by the controller controlling the receiver and thebattery-backup power supply to enter a standby mode of operation inwhich the receiver scans for a CONNECT message indicating an incomingcall, the controller controlling the sleep mode and the standby mode ofoperations based on a frame count that is generated from anidentification number of the remote unit.
 17. The remote unit accordingto claim 16 , wherein the receiver scans for a connect message for apredetermined number of subframes of a TDD timing structure.
 18. Theremote unit according to claim 17 , wherein the predetermined number ofsubframes equals
 3. 19. The remote unit according to claim 17 , whereinwhen the remote unit enters the standby mode, the remote unit reacquiressynchronization to the TDD timing structure.
 20. The remote unitaccording to claim 19 , wherein the remote unit reacquiressynchronization to the TDD timing structure in about 34 subframes. 21.The remote unit according to claim 19 , wherein the remote unit scansfor a CONNECT message at a subframe that is related to an identificationnumber of the remote unit.
 22. A method for reducing power consumptionof a remote unit in a PWAN system, comprising the steps of: powering aremote unit using a battery backup power supply when an AC power supplyfails at the remote unit; entering a sleep mode of operation at theremote unit, the sleep mode having a reduced power consumption for thebattery backup power supply; entering a standby mode of operation at theremote unit a predetermined period of time after entering the sleep modeof operation scanning for a CONNECT message indicating an incoming callfor the remote unit; and reentering the sleep mode of operation when noCONNECT message is received.
 23. The method according to claim 22 ,further comprising the step of synchronizing the remote unit to a TDDtiming structure before the step of entering the standby mode ofoperation.
 24. The method according to claim 23 , wherein thepredetermined period of time is a predetermined number of subframesafter a boundary subframe of the TDD timing structure.
 25. The methodaccording to claim 24 , wherein the predetermined number of subframes isbased on an identification number of the remote unit.